<dxhe:ASPxHtmlEditor ID="ASPxHtmlEditor1" runat="server">

    <Toolbars>

        <dxhe:CustomToolbar>

            <Items>

                <dxhe:CustomToolbarButton CommandName="CustomSpellChecking"

                    Text="CustomSpellChecking">

                </dxhe:CustomToolbarButton>

            </Items>

        </dxhe:CustomToolbar>

    </Toolbars>

    <ClientSideEvents CustomCommand="function(s, e) { OnCustomCommand(s, e.commandName); }" />

</dxhe:ASPxHtmlEditor>

<script type="text/javascript">

    function OnCustomCommand(htmlEditor, commandName) {

        if (commandName == "CustomSpellChecking") {

            CustomSpellChecking(htmlEditor);

        }

    }

    function CustomSpellChecking(htmlEditor) {

        var html = htmlEditor.GetHtml();

        if (html != "<p></p>") // some conditions

            htmlEditor.ExecuteCommand(ASPxClientCommandConsts.CHECKSPELLING_COMMAND);

    }

</script>